Abstract

There is anatomical evidence that there are two main populations of preganglionic neurones that project to the cardiac vagal branch in several species including rat, cat and dog. One population originates in the ventrolateral nucleus ambiguus (NA), and comprises neurones with small myelinated axons (B-fibres) (McAllen and Spyer, 1978). The other group originates in the dorsal vagal motor nucleus (DVMN) and comprises mainly of slowly conducting, unmyelinated axons (C- fibres) (Fordet al.,1990). The B-fibre group has been shown to display a powerful respiratory-related component to their activity (McAllen and Spyer, 1978), whereas it has been reported that the majority of C-fibre vagal efferents located in the DVMN show no modulation in phase with respiration (Joneset al.,1998).